1. Field of the Invention
The present invention relates generally to heating with electromagnetic wave energy in a conductive cavity, and more particularly, pertains to a directional rotating antenna rotated about an axis axis of one wall of a microwave oven cavity of a microwave oven which provides uniform energy distribution and a consistent cooking pattern of food products.
2. Description of the Prior Art
Prior art microwave ovens suffer from nonuniform energy distribution, and more particularly, nonuniform heating patterns depending upon the type of particular product, which usually is food, being heated. The degree of the nonuniform heating pattern relates to the specific type of product being heated. The nonuniform heating pattern occurs because of unequal distribution of microwave energy coupled into a conductive cavity of a microwave oven from a source of microwave power such as a magnetron, and from the reflections of microwave energy from the product within the microwave oven cavity and the conductive walls framing the microwave oven cavity. Multiple reflections within the conductive microwave oven cavity occur and produce configurations of the electromagnetic fields referred to as modes. These reflections cause constructive and destructive interference at and in different parts of the product being heated, and therefore, result in hot areas intermixed with cold areas. Where the product is food, the result is overcooked areas of the food intermixed with undercooked areas of the food. In some food products, the cooking time and thus energy consumption may be increased to fill in the undercooked areas as any overcooked areas may be less objectionable.
Some food products which have been particularly difficult to cook in the prior art microwave ovens include yeast products such as breads; baked products such as cakes and pies; scattered products such as cookies, appetizers, and hors d'oeuvres, and; egg dishes such as custards and quiches. All of these types of food products when cooked in many of the prior art microwave ovens have exhibited overcooked areas intermixed with undercooked areas leaving much to be desired in the cuisine of the consuming gourmet. The cooking of other types of food products require cooking at low power in addition to manual cooking manipulations in an attempt to obtain even cooking which has not always been possible to achieve with the prior art microwave ovens.
The principal process for improving nonuniform heating patterns in the prior art microwave ovens has been mode stirring which attempts to randomize reflections by introducing a time-varying scattering of the microwave energy. Mode stirring can be characterized as a change of energy distribution of the different modes in the microwave oven cavity. Most systems utilizing mode stirrers position the mode stirrer at the waveguide-microwave oven cavity junction where the waveguide couples microwave energy from the microwave power source to the microwave oven cavity and require turning of the product such as food. The mode stirrers are not as effective since hot spots intermixed with cold spots continue to exist in the heated product. Hot spots intermixed with cold spots are also a function of the nonuniformity of the product such as fat versus lean, ice versus water, and other density factors.
Another process for improving the nonuniform heating patterns in the prior art microwave ovens has been utilizing a turntable within the microwave oven cavity to rotate the product about a center axis of the microwave oven cavity through an uneven energy pattern. The turntable wastes useful oven space and requires additional complex mechanical components. Sometimes, the turntables compliment microwave ovens having mode stirrers. Regardless of the process of utilizing a turntable or a turntable and mode stirrer, hot spots intermixed with cold spots still exist in the heated product.
A further process for improving the nonuniform heating patterns in the prior art microwave ovens has been utilizing rotatable antennas or exciters within the microwave oven cavity. The prior art rotatable antennas or exciters have failed to provide uniform heating patterns in the product being heated. An example of representative prior art patents disclosing a rotatable antenna are U.S. Pat. Nos. 4,028,519, 4,028,520 and 4,028,521.
Prior art directional antennas for uses not associated with microwave ovens have been described in many configurations. Those of compact size employing linear radiating elements are usually assembled from rows of resonant dipoles fed at their centers by balanced transmission lines. Such antennas may be excited from a waveguide through the use of a rotating joint and an unbalanced-to-balanced line balun transformer. The energy is then routed to the antenna elements by a balanced transmission line which consists of two symmetrical conductors. This approach would be very difficult to implement in a consumer microwave oven where its complexity would increase overall costs, especially when a substantial power must be handled with poor and unpredictable loads. Furthermore, such a structure would be very heavy and mechanically difficult to rotate by air or motor.
The present invention provides a microwave oven having a uniform energy distribution and a consistent heating pattern that overcomes many of the disadvantages of the prior art microwave ovens.